Field of the Invention
The present invention relates to a worm wheel of a reduction gear for an electric power steering, and more particularly, to a worm wheel of a reduction gear for an electric power steering, in which coupling surfaces of a boss part and a tooth-shaped part are formed to have a curved shape such as a wave pattern and are coupled in a male-female configuration, so that a contact area therebetween is increased and a curve is formed along a rotational direction, thus increasing a coupling strength.
Description of the Related Art
In general, an electric power steering (EPS) system makes a steering effort of a steering wheel soften during parking or low-speed driving according to a traveling speed of a vehicle due to motor power.
The EPS system provides a driver with a heavy steering sense for stability during a high speed driving and allows a driver to perform a rapid steering in case of emergency, thus providing a driver with an optimal steering condition.
As such, the EPS system allows a driver to perform a steering operation even with a small force regardless of a steering gear ratio and can absorb a shock generated due to unevenness of a road and prevent the shock from being transferred to a steering wheel.
In addition, recently, according to a driving condition detected by a vehicle speed sensor, a steering torque sensor, or the like, an electronic control unit (ECU) is used to operate a motor to provide a driver with a light and convenient steering sense during a low speed driving and a heavy steering sense and a stable directionality during a high speed driving, and to allow a driver to perform a rapid and instant steering in case of emergency. In other words, it is a current trend to employ an EPS system in order to provide drivers with optimal steering conditions.
Examples of the EPS system include an electro-hydraulic power steering (EHPS) system and a motor driven power steering (MDPS) system. Unlike a hydraulic system that forms a hydraulic pressure in pump and assists power, the MDPS system is configured to assist steering power by using a torque of a motor and assist steering force in such a manner that a worm shaft connected to a motor for generating power rotates a worm wheel connected to a steering shaft.
The worm wheel includes a boss part and a tooth-shaped part. The boss part has a hollow portion formed in an axial direction such that the worm wheel is connected to the steering shaft. The tooth-shaped part is connected to a periphery of the boss part and is engaged with the worm shaft.
However, in the conventional worm wheel, when the boss part and the tooth-shaped part are coupling-molded, a coupling portion of the boss part and the tooth-shaped part is shrunk to a predetermined depth. Thus, it is feared that a gap will be formed between a coupling surface between the boss part and the tooth-shaped part.
As the prior art document related to the present invention, Korean Patent Application Publication No. 10-2007-0051180 (May 17, 2007) discloses a power steering system.